1. Field of the Invention
The present invention relates to a light responsive circuit for generating a light representative output signal which is used for camera exposure control or indication of measured light, and more particularly pertains to a light responsive circuit adapted to measure natural or ambient light and artificial flash light and including a logarithmic conversion circuit.
2. Prior Art
It is known to provide a light responsive circuit with a logarithmic conversion circuit for the convenience of treatment of electric signals. The logarithmic conversion circuit usually consists of a logarithmic compression circuit which is coupled with a photocell and generate an electric signal proportional to the logarithm of measured light intensity, and a logarithmic expansion circuit for generating another electric signal proportional to the signal from the logarithmic compression circuit usually modified while the signal is transmitted. The modification may often be an exposure calculation wherein an electric signal representative of at least one of a number of exposure factors, such as film sensitivity or diaphragm aperture value, is added to or subtracted from the light representative and logarithmically compressed signal from the logarithmic compression circuit. As the compressed signal varies in arithmetical progression whereas the light signal varies exponentially or logarithmically, the electric exposure calculation with the compressed signal requires a considerably narrow range for the electric signals to be treated processed, and also requires only addition and subtraction of signals. This calculation system can be compared to exposure calculation according to the APEX system. Thus such logarithmic conversion circuitry has been popularly used in exposure control circuits with or without a light signal storing means as well as in exposure meters.
It is also known to measure both ambient light and flash light with a common photoelectric circuit provided with a photocell which is highly responsive in a short time to the incident light. Such a light measuring system has been adopted, for example, in an exposure control system wherein a highly responsive photocell disposed in a camera body to receive scene light passing through the camera objective lens and reflected from the focal plane shutter surface and/or film surface is associated with a control circuit which generates a control signal to be delivered not only to a magnetic device for controlling exposure termination but also to a flash control circuit for controlling duration of flash firing. Another example of a device employing such a light responsive circuit may be a flash meter for providing an indication of optimum exposure conditions for flash photography as well as for ordinary photography with ambient light. These common photoelectric circuit systems have merit in that a single photoelectric circuit is commonly used for dual purposes and another photoelectric circuit which otherwise would be required for one of the purposes is not necessary.
Thus, it is appreciated that those skilled in the art may consider employment of a logarithmic conversion circuit in the common photoelectric circuit system. However, this combination involves a serious problem due to characteristics of the semiconductive element included in the conversion circuit.
If the light intensity to be measured is so low as to provide the photocell with very little output, then the logarithmically compressed signal does not accurately correspond to the output of the photocell due to current leakage to the base plate on which the circuit elements are disposed, the affect of bias current on the amplifier for the photocell, nonlinearity of the logarithmic compression element for such low input, the characteristics of the calculation circuit and so forth. However, if the light intensity to be measured is so high as to provide the photocell with a large output, then the input to the logarithmic expansion element will be so high that the output of the expansion element is saturated and its linearity is lost, although the output of the logarithmic compression element has a satisfactory linearity for its high input.
Now, further detailed discussion is presented with respect to the case where the above mentioned light responsive circuit including the logarithmic conversion circuit is employed in an automatic exposure control system in which the output of a control circuit responsive to a single photoelectric circuit is utilized not only for the exposure control but also for the control of flash light duration.
Referring to FIG. 1, which shows the general construction of such an automatic exposure control system schematically, photocell PD is disposed within a camera body to receive the light from an object to be photographed, such light passing through the camera objective and the objective diaphragm aperture AP and reflected from the film surface and/or the surface of a focal plane shutter curtain. Alternatively, photocell PD is mounted on the camera body to receive the object light directly through an aperture AP of a light control member interlocked with the objective diaphragm. In both cases, the photocell can receive the object light during shutter operation, i.e. film exposure. Block 1 includes a logarithmic compression circuit for providing a compressed signal proportional to the logarithm of the light intensity incident on the photocell, and a calculation circuit for adding to or subtracting from the compressed signal an electric quantity, e.g. voltage, applied from an exposure factor setting device 3 for providing an electric quantity representative of a set exposure factor such as film sensitivity. Thus the block 1 circuitry generates an output signal as a result of a combination of the compressed signal and the electric quantity, i.e. as a sum of a set exposure factor representative signal and the logarithmically compressed signal, or a remainder of the subtraction therebetween. Block 2 includes a logarithmic expansion circuit for generating an expanded signal proportional to the anti-logarithm of the input signal from the block 1 circuitry, an integrating member for integrating the output of the logarithmic expansion circuit, i.e. the expanded signal, and a threshold circuit for generating a control signal to actuate electromagnet Mg by deenergizing it when the output of the integrating circuit reaches a given threshold level.
If photocell PD is highly responsive to the light incident thereon (a silicon photodiode is known to satisfy such a requirement), the control signal from the block 2 circuitry may also be utilized to control the flash light duration such that a switching element such as a silicon control rectifier (SCR) connected in series with the flash tube is turned off, or a bypass element connected in parallel with the tube is turned on, in response to the control signal so that the flash tube which has been fired in response to shutter opening may be turned off upon the generation of the control signal.
However, such flash control in response to the exposure control signal entails the following problems. The logarithmic expansion circuit ordinarily includes a logarithmic expansion transistor which generates an expanded current that is proportional to the anti-logarithm of its base voltage as an input signal of the block 2 circuitry. The expansion transistor which is included in an integrated circuit loses its linearity between its base-emitter voltage and collector current (expanded current) with the collector current being above 800 .mu.A. (see FIG. 2). Accordingly, when the brightness of an object to be photographed is so high as to require the collector current above that limit, error is likely to be incurred in the exposure control. This will occur frequently in case of flash photography wherein the object to be photographed is illuminated by a flash light of extremely high intensity. However, for the case where the object brightness is very low, and accordingly the expanded current also very low, the lower limit of the expanded current should be about 100 nA so that the leakage current of the integrating member, such as a capacitor, which current is estimated to be several nA, is negligible as compared with the expanded current, i.e. collector current of the expansion transistor. Thus, for the characteristic curve shown in FIG. 2, the range of the collector current available for the exposure control should have an upper limit of 800 .mu.A and a lower limit of 100 nA.
Assuming that the light measuring circuit including the photocell PD, block 1 circuitry and the exposure factor setting device 3, and the exposure control circuit including the block 2 circuitry and the electromagnet Mg are so designed that the expanded currents at the upper and lower limits, i.e. of the value 800 .mu.A and 100 nA, respectively, correspond respectively to 1/1000 second and 8 seconds of exposure time for daylight photography without flash light, then the difference between the output voltages of the light measuring circuit corresponding to the upper and lower limits of the exposure time will be 234 mV.
In case of flash photography, the photocell is to receive light of extremely high intensity so that the minimum time period to be established by the integrating member for controlling the duration while the flash tube is being fired is estimated to be as short as 1/64000 sec. which corresponds to a base-emitter voltage of 108 mV larger than that corresponding to the exposure time of 1/1000 sec. and which requires 52 mA of collector current as determined on the dotted line extended from the linear portion of the real line in FIG. 2. However, the collector current actually obtained for the base-emitter voltage corresponding to the flash light duration of 1/64000 sec., is smaller than 108 mV due to saturation of the collector current as seen on the real line in the range above 800 .mu.A of the collector current in FIG. 2. As a result, the flash light duration will be longer than 1/64000 sec. to cause over-exposure.